Composite plastic resins whose curing is ensured by photopolymerization are increasingly being employed as materials for filling, adhesive bonding, coating or impressing, and for other uses where their ease and quickness of use may be a decisive advantage.
In particular, in the field of restorative dentistry and of dental prostheses, such photopolymerizable resins are more and more frequently employed instead of materials of metallic type.
Similarly, in the field of bone surgery, these materials are increasingly employed for bone restoration or for the manufacture of prostheses.
These composite plastic resins generally consist of monomers or of a prepolymer which are mixed with a diluent to adjust its fluidity and with various additives promoting the photopolymerization. To this mixture, which forms the matrix of the composite resin, there is added a variable proportion, depending on the desired characteristics, of filler consisting of very fine grains or fibers of quartz, silica, and the like, intended to increase the mechanical properties (hardness, strength) of the composite resin.
In all these applications, and in particular when the polymerization must be performed in situ, the flexibility and the speediness of photopolymerization, i.e., polymerization under the effect of luminous radiation, constitute valuable advantages and make the practitioner's task easier.
The use of luminous sources of high photopolymerization performance, insofar as photopolymerization is concerned, particular of the argon laser, has made it possible to broaden the field of application and the practical possibilities of the use of this process.
However, the polymerization of composite resins is always accompanied by a volume shrinkage which results in major disadvantages and limits the performance of polymerizable resins in many applications.
This is so in the case of dental or bone restoration, where polymerization shrinkage can give rise to stresses which are capable of inducing pain in the patient or causing a certain deterioration of the prostheses, such as fissures, opening the way to bacterial infections.
In the case of other applications, and in particular in the case of the manufacture or of the assembly of highly precise industrial components, polymerization shrinkage makes it difficult to maintain accurate dimensions and gives rise to the appearance of fissures or of separations, in particular in the connecting regions between the components.